The present application relates to information encoding for transmission over noisy channels and storage, and more particularly to error resilient encoding.
Two common approaches to the mitigation of errors arising during the transmission of data over noisy channels exist: Automatic Retransmission Request (ARQ) and Forward Error Correction (FEC). ARQ type of mitigation typically would not be feasible in multicast or real-time applications such as video because of intolerable time delays or a lack of feedback channel. In such cases, a decoder can only decode the error corrupted bitstream, protected to an extent by error correction encoding, and must create from such bitstream. FEC provides mitigation by error correcting codes (e.g., Reed-Solomon). However, uncorrectable errors require further mitigated approaches.
In general, commonly used video compression methods have block-based motion compensation to remove temporal redundancy. Motion compensation methods encode only (macro)block motion vectors and the corresponding quantized residuals (texture); and variable length coding (VLC) of the motion vectors and residual increases coding efficiency. However, variable length coding often are highly susceptible to transmission channel errors and a decoder easily loses synchronization with the encoder when uncorrectable errors arise. The predictive coding methods, such as motion compensation, make matters much worse because the errors in one video frame quickly propagate across the entire video sequence and rapidly degrade the decoded video quality.
The typical approach of such block-based video compression methods to uncorrectable errors includes the steps of error detection (e.g., out-of-range motion vectors, invalid VLC table entry, or invalid number of residuals in a block), resynchronization of the decoder with the encoder, and error concealment by repetition of previously transmitted correct data in place of the uncorrectable data. For example, video compressed using MPEG1-2 has a resynchronization marker (start code) at the start of each slice of macroblocks (MBs) of a frame, and an uncorrectable error results in all of the data between correctly decoded resynchronization markers being discarded. This implies degradation in quality of the video stream, especially for predictive compression methods such as MPEG.
These video compression and decompression methods may be implemented on special integrated circuits or on programmable digital signal processors or microprocessors.